With the ever increasing growth of automated systems used in various types of industrial and medical systems, there is a need for new and improved sensors and signal processing apparatus for monitoring movements related to force, torque, speed, acceleration, contraction, expansion, rotation, deformation, displacement, and the like. There is also a need to miniaturize such sensors to make measurements not otherwise possible with large and bulky sensors. For example, when monitoring the displacements of small or fragile items, or when monitoring deformations of flexible membranes such as skin, it is important that the sensor mass, its attachments, its electrical connections, and its operation, do not interfere with the movements being monitored to the extent that might otherwise significantly impact the accuracy of the measurements.
Such sensors and signal processing apparatus should preferably be subject to low manufacturing costs, not require high tolerance fits for moving parts, provide a sizable range of linear or tailored operation, and yet be relatively rugged.
Sensors of the prior art and inductors therefore for monitoring movements take the form of standard type transformers with multiple coils of turns of wires wrapped to extend longitudinally over a common axis, wherein the coils are being positioned adjacent to, or on top of, the other, in the form of a tubular unit. An armature extends along the axis into the tubular unit for movement therein relative to the coils and the magnetic flux from one coil flows along the armature to the other coil.
An example of a transformer type sensor is a linear variable differential transformer (LVDT) type sensor. A LVDT sensor is disclosed in the U.S. Pat. No. 5,216,364, issued on Jun. 1, 1993, entitled “Variable Transformer Position Sensor” that includes mechanical structures for use in automotive shock absorbers.
Miniaturized transformer type sensors based on the LVDT technology are disclosed in catalog publications by Micro-Epsilon entitled “Inductive Displacement Sensors and Linear Gaging Sensors,” and by Singer Instruments and Control, Ltd. entitled “SM Series LVTD.”
The U.S. Pat. No. 5,497,147, issued on Mar. 5, 1996, and entitled “Differential Variable Reluctance Transducer,” and U.S. Pat. No. 5,777,467, issued on Jul. 7, 1998, and entitled “Miniaturized Displacement Transducer Assembly,” and publication by MicroStrain entitled “Differential Variable Reluctance Transducer” (DVTR), disclose transformer type sensors
The U.S. Pat. No. 3,891,918, issued on Jun. 24, 1975, and entitled “Linear Displacement Transducer Utilizing An Oscillator Whose Average Period Varies as a Linear Function of the Displacement,” also includes a transformer type sensor.
Transformer type sensors are disclosed in the U.S. Pat. No. 5,216,364, and the Micro-Epsilon and the Singer publications, and in a publication by Analog Devices entitled “LVDT Signal Conditioner AD598 (Rev A)” and a publication by David S. Nyce of Revolution Sensor Company entitled “The LVDT a Simple and Accurate Position Sensor” dated August 2005.
In the field of medicine there is continual research and development for the design of new equipment for monitoring body changes to measure internal physiological properties, such as the chest for problems dealing with sleep apnea and the abdomen for pregnancy labors. The present solutions require the use of belt and/or vest type sensing arrangements. For sleep apnea the vests and belts surround the chest torso such as disclosed in many United States Patents, of which the following are sample patents: U.S. Pat. No. 5,329,932, issued Jul. 19, 1994, entitled “Method of and Apparatus for Monitoring Respiration and Conductive Composition Used Therewith,” U.S. Pat. No. 6,142,953, issued Nov. 7, 2000, entitled “Respiratory Inductive Plethysmography Band Transducer,” U.S. Pat. No. 6,413,225, issued Jul. 2, 2002, entitled “Quantitative Calibration of Breathing Monitors with Transducers Placed on Both Rib Cage and Abdomen,” U.S. Pat. No. 6,461,307, issued Oct. 8, 2002, entitled “Disposable Sensor for Measuring Respiration,” and U.S. Pat. No. 6,551,252, issued Apr. 22, 2003, entitled “Systems and Method for Ambulatory Monitoring of Physiological Signs.” For pregnancy labors, the belts surround the abdomen such as disclosed in a Philips Medical Systems Nederland B. V. publication entitled “FM-2 Antepartum Portable Fetal Monitor.” Each of these apparatus is bulky and as a result may be relatively uncomfortable to wear for extended periods of time, particularly if required to wear them overnight. Furthermore, although the apparatus may be portable, they are cumbersome, and may interfere with daily activities, and sleep.
There is a need to replace these massive and cumbersome belts and vest apparatus that encircle the body or cover large portions of the torso, and avoid short-term and long-term patient discomfort that may accompany their use. The apparatus should preferably be attached and worn with minimal discomfort, allowing the patient a significant amount of freedom of movement without impacting the tests underway. The apparatus should also preferably have a high degree of sensitivity to allow the equipment to detect small changes, particularly when testing infants, and be capable of continued operation as the patient changes positions.
The Q (quality factor) of a coil is defined as the ratio of the inductive reactance to the resistance of the transformer wire wound type coil at a given frequency. Q is a measure of the efficiency of storing energy; the higher the Q the more efficient the coil. To increase the Q in the abovementioned transformer type sensors, either the frequency applied to the sensors is to be increased, or the sensor inductive reactance increased (by the number of coil wire turns squared), or the sensor internal resistance is decreased. However, the miniaturization of the wire wound transformer type sensors do not scale well due to Q restraints. As the dimensions of these sensors are decreased, primarily by reducing the size of the wire, the internal resistance of the sensor coils increases significantly. It would be advantageous if the sensor design were not limited by Q restraints.
The use of commercial type strain gauges to measure deformations or the body was found unworkable in that their attachment of such strain gauges onto the body interfered with the movements of the part of the skin to which the gauges were attached rendering their use questionable.
In addition, it would advantageous if the monitoring apparatus was completely portable and adaptable for use over a wide variety of portions of the body for observing a wide variety of physiological problems.
Further it would be advantageous if the sensor could be subject to miniaturization for use with miniaturized monitoring circuitry, including radio, infrared, etc; for transmission of data to remote locations, with a readily detachable connection in between so that the low cost sensors can be discarded and the monitoring circuit reused.